全文获取类型
收费全文 | 45533篇 |
免费 | 4282篇 |
国内免费 | 34篇 |
出版年
2021年 | 618篇 |
2020年 | 349篇 |
2019年 | 426篇 |
2018年 | 578篇 |
2017年 | 524篇 |
2016年 | 948篇 |
2015年 | 1637篇 |
2014年 | 1790篇 |
2013年 | 2257篇 |
2012年 | 2824篇 |
2011年 | 2916篇 |
2010年 | 1915篇 |
2009年 | 1831篇 |
2008年 | 2674篇 |
2007年 | 2662篇 |
2006年 | 2615篇 |
2005年 | 2579篇 |
2004年 | 2508篇 |
2003年 | 2462篇 |
2002年 | 2406篇 |
2001年 | 540篇 |
2000年 | 433篇 |
1999年 | 612篇 |
1998年 | 706篇 |
1997年 | 499篇 |
1996年 | 456篇 |
1995年 | 433篇 |
1994年 | 413篇 |
1993年 | 427篇 |
1992年 | 366篇 |
1991年 | 338篇 |
1990年 | 353篇 |
1989年 | 266篇 |
1988年 | 313篇 |
1987年 | 299篇 |
1986年 | 292篇 |
1985年 | 359篇 |
1984年 | 397篇 |
1983年 | 360篇 |
1982年 | 452篇 |
1981年 | 442篇 |
1980年 | 424篇 |
1979年 | 276篇 |
1978年 | 309篇 |
1977年 | 273篇 |
1976年 | 284篇 |
1975年 | 220篇 |
1974年 | 289篇 |
1973年 | 264篇 |
1972年 | 173篇 |
排序方式: 共有10000条查询结果,搜索用时 16 毫秒
951.
Cellular accumulation of ascorbic acid was investigated in vitro in distal intestinal mucosa of guinea pig. With 14C-ascorbic acid present at 8 μM/L in the bathing media, tissue/media (T/M) concentration ratios of at least 5 were routinely achieved. Recently absorbed ascorbic acid appeared to be free in solution in the cellular fluid in that it diffused from tissue exposed to poisons with a disappearance half-time of approximately 10 minutes. Ascorbic acid uptake was highly dependent on the presence of sodium in the bathing media; total Tris substitution resulted in a 97% decrease in uptake. Also, metabolically depleted tissue did not accumulate ascorbic acid against a concentration gradient. Uptake of 14C-ascorbic acid from a bathing solution concentration of 8 μM/L was reduced 67% in the presence of 0.8 mM/L nonlabeled ascorbic acid. Recently absorbed 14C-ascorbic acid moved more rapidly back into the lumen when the luminal solution contained nonlabeled ascorbic acid (5 mM) than when it contained mannitol (5mM). This demonstration of counter transport substantiates a carrier mechanism in the brush border. 相似文献
952.
We examined the effects of isopropanol (ISOP) pretreatment on the metabolism of 14CCl4 to 14CO2 and CHCl3 exhaled in the breath, to 14C metabolite excreted in 24 hr urine and feces from 0 to 24 hr, and to 14C metabolite bound to liver at 24 hr. Fasted male rats were given 0.1 or 2.0 mmoles 14CCl4/kg. ISOP pretreatment, which markedly enhanced the hepatotoxicity of CCl4, selectively enhanced the rate and total extent of 14CO2 and CHCl3 metabolite exhalation. The pathways of CCl4 metabolism leading to CO2 and CHCl3 metabolite formation may be more relevant to the hepatotoxicity of CCl4 than the pathways leading to urinarym fecal or covalently bound metabolites. 相似文献
953.
Brassinosteroid-Induced Bending of the Leaf Lamina of Dwarf Rice Seedlings: An Auxin-Mediated Phenomenon 总被引:7,自引:0,他引:7
A synthetic brassinosteroid (BR), 2,3,22ß,23ß-tetrahydroxy-24ß-methyl-B-homo7-oxa-5-cholestan-6-one, an isomer of the growth promoter brassinolide,when applied to seedlings of dwarf rice Oryza sativa var. Tan-ginbozuand Waito-C, induced a significant bending of the second leaflamina at 100 ng/plant and higher dosages. Promotion of thesecond leaf sheath elongation, the characteristic response ofdwarf rice varieties to gibberellins, was significantly butmodestly enhanced by BR at a dosage of 10,000 ng/plant, fiveorders of magnitude higher than the minimal dosage responseto GA3. Gibberellin A3 had no significant effect on the bendingof the second leaf lamina, nor did any synergism exist betweenBR and GA3 in leaf lamina bending or leaf sheath elongation.Neither ethylene nor (2-chloroethyl)phosphonic acid (ethephon)caused the bending of the second leaf lamina, and neither synergizedthe BR effect. However, IAA and -naphthaleneacetic acid causedsignificant bending at 5,000 ng/plant, and both auxins significantlysynergized the effect of BR on the bending, IAA being effectiveat 500 ng/ plant in this regard. The antiauxins, 2,3,5-triiodobenzoicacid (TIBA) and -(p-chlorophenoxy)isobutyric acid (PCIB) completelynullified both the BR-induced bending and the BR$IAA-synergizedbending. The BR-induced bending response may thus be mediatedthrough endogenous auxin. (Received May 11, 1982; Accepted August 25, 1982) 相似文献
954.
Summary We have developed a miniature silver-silver chloride electrode. The outer diameter of the electrodes averaged 22 m and the input resistance 8.8 k. Since the core of the electrode is a glass fiber, the problem of the extreme malleability of a small diameter silver fiber is circumvented. The properties of the electrode permit us to insert it into short (600 m) fragments of the amphibian collecting duct while they are being perfusedin vitro. The passage of currents in the range of 0 to 6×10–8 amperes allowed us to voltage clamp the nephron fragment between +20 and –20 mV. The current-voltage plots are linear over this range. Two lines of evidence suggest that the voltage clamp is homogeneous. First, the voltage measured at the perfusion end during a voltage-clamp experiment of the tubule is not significantly different from that measured at the collecting end. Secondly, the specific resistance of collecting ducts estimated from the core conductor analysis is 3.3±0.8×104 cm, a value not significantly different from that computed from the current-voltage plots as determined with the Ag–AgCl electrode, 3.0±0.5×104 cm. This method permits precise control of both the ionic and electrical gradients across fragments of the amphibian collecting duct. 相似文献
955.
956.
Photosynthetically fixed 14C was analyzed in various chemical fractions from leaves and stems of cottonwood (Populus deltoides Bartr. ex. Marsh.) during dormancy induction. Dormancy was induced by 8-h photoperiods and 20/14°C temperature regimes. Within 4 weeks under short days, terminal buds were set and leaf expansion and stem elongation had stopped. 14C2 was fed to a leaf at Leaf Plastochron Index 7 for 30 min. Either after this 30 min feeding period or after a 48-h translocation period the plants were sampled, freeze-dried, extracted and analyzed for14C. 14C-fixation decreased during dormancy induction from 60% to 17% of the 3.7 MBq 14C applied at 0 week and 8 weeks, respectively. Percentage distribution of 14C in chemical fractions of source leaves reflected leaf age and translocation inhibition. In rapidly growing plants, considerable 14C was incorporated into leaf protein while most of the soluble14C-sugars were either metabolized or translocated out of the leaf. After terminal bud set, the percentage of 14C in the protein and residue fractions decreased rapidly and that in the sugar fraction increased. Percent distribution in stems closely reflected changing metabolic pathways of carbon flow as influenced by dormancy induction. For example, the 14C in structural carbohydrates decreased in 5 weeks under short days from 65 to less than 10% of the 14C recovered in the chemical fractions, thus indicating cambium inhibition. At the same time the percentage of 14C in starch and sugar increased indicating storage. Short term (after 30 min) incorporation of 14C into the protein and starch fractions of leaves changed relatively little throughout the 8-week induction period. In contrast the turnover rates of these fractions (14C present after 48 h) increased considerably after active growth of the whole plant stopped. 相似文献
957.
Peter J. Davies Eve Emshwiller Thomas J. Gianfagna William M. Proebsting Masana Noma Richard P. Pharis 《Planta》1982,154(3):266-272
The gibberellins (GAs) of both vegetative (leaves and stems) and reproductive (pods and seeds) tissue of the G2 strain of peas Pisum sativum L. were characterized in purified extracts by a combination of sequential silicic-acid partition column chromatography, and gas chromatography-mass spectrometry. Gibberellins A19, A20, A29 and an A29 catabolite were identified in both types of tissue. Gibberellins A9, A17 and A44 were also found in pods and seeds.Abbreviations FID
Ilame ionization detector
- GA(s)
gibberellin(s)
- GC
gas chromatograph(y)
- HPLC
high performance liquid chromatograph(y)
- LD
long day
- MS
mass spectrum(a) or mass spectrometer(ry)
- SD
short day 相似文献
958.
Lipoamide dehydrogenase (NADH:lipoamide oxidoreductase EC 1.6.4.3) has been isolated from Ascaris suum muscle mitochondria. This activity has been purified to apparent homogeneity from both the pyruvate dehydrogenase complex and from 150,000g mitochondrial supernatants which were devoid of pyruvate dehydrogenase complex activity. The enzymes from both sources exhibited similar kinetic, catalytic, and regulatory properties and appear to be identical as judged by polyacrylamide gel electrophoresis. The native enzyme acts as a dimer, containing 2 mol of FAD, and has a subunit molecular weight of 54,000, as estimated by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and gel chromatography. The enzyme also possesses substantial NADH:NAD+ transhydrogenase activity. Heat denaturation and differential solubilization experiments imply that the transhydrogenase activity previously reported is, in fact, associated with the lipoamide dehydrogenase moiety of the Ascaris pyruvate dehydrogenase complex. Whether or not this activity functions physiologically in hydride ion translocation, as previously suggested, remains to be demonstrated. 相似文献
959.
The mixed culture ‘Chloropseudomonas ethylica’ strain 2K has been grown on a medium which enhanced the yield of cytochrome c-551.5 from Desulfuromonas acetoxidans. The cytochrome was purified to homogeneity and an isoelectric point of 8.40 was determined. A determination of the amide content indicated that the cytochrome contains two more amides than previously reported. 相似文献
960.
Internodal and whorl (branch) cells of the green alga,Chara corallina Klein ex Willd., em. R.D.W., were studied with the extracellular vibrating probe for measuring transmembrane ion currents, and with an extracellular pH microprobe for measuring the surface pH profile. Bands of positive inward current (OH- efflux) 1–3 mm wide were separated by wider bands of outward current (HCO
3
-
influx) along the length of the cell. The measured peaks of inward current ranged from 20 to 60 A cm-2 (98 m from the cell surface) which would correspond to a surface ionic flux of 270–800 pmol cm-2 s-1. The peaks of outward current (HCO
3
-
influx) ranged from 10 to 30 A cm-2 which would correspond to a surface ionic flux of 140–400 pmol cm-2 s-1. The inward current bands matched the regions of surface alkalinity very well. The outward current (HCO
3
-
influx) was reduced at least 10-fold in low-HCO
3
-
medium, with a commensurate readjustment in the strength and pattern of inward current (OH- efflux). (Although these experiments involved a manipulation of the external pH, it is felt that the main adjustment in current patterns was in response to the reduction in exogenous HCO
3
-
). The presence of the vibrating probe perturbed the inward current region when vibrating with a 26-m amplitude, but this perturbation was eliminated when a 7-m amplitude was used. The perturbation was usually observed as a reduction in the number of inward current peaks with an increase (approximate doubling) in the amplitudes of the one or two remaining peaks. Both the inward and outward currents were light-dependent, falling off within seconds of light removal. 相似文献